Use the block to support the I-shaped cast iron straight edge. Choose the proper bridge plate according to the length of the working surface of the cast iron straight edge. Then, place the bridge plate on one end of the straight edge, and fix the reflector or gradienter onto the plate. According to the span, gradually move the bridge plate from one end to the other end of the straight edge. Every time after you have moved the plate a span, you should get the reading from the autocollimator with the division value of 1″, or from the electronic level meter whose division value is of 0.001mm/m. The difference between the maximum and minimum values is the working surface straightness error of the cast iron straight edge.

The factors like environment temperature, humidity, vibration, etc., will affect the flatness precision of the cast iron surface plate. It is important to control the application environmental aspects. But to obtain the ideal precision in the daily application of the cast iron surface plate, you also need to pay more attention to the maintenance and proper use of the surface plate. The improper use will result in the measurement error. Meanwhile, the insufficient maintenance will lower the surface precision. Even worse, the quality of the inspected product will be affected accordingly. Therefore, you are required to mind the following items related to the use and maintenance of cast iron surface plate.

Because of its own inner stress, the cast iron surface plate may deform under the function of stress, humidity, or other relevant factors. Generally, it should have the surface precision inspection after having been used for half a year. Repairing is required once the precision is proved to be unqualified.
After having been used for a long time, the cast iron surface plate will be possibly lowered in flatness or degree of finish. If there is any thump caused breakage or severe deformation, you should replace the surface plate with a new one.

The bottom part of the cast iron surface plate should be treated with the thorough cleaning process. And you should paint it with red anti-rust paint or coatings. Also, the side surface needs cleaning process prior to the rough processing. After the leveling, it should be coated with blue, green, or yellow paint.
The cleaning process could be manually or mechanically accomplished. The manual cleaning process requires low cost but some parts can't be cleaned completely. Instead, the mechanical cleaning process can make the cast iron surface plate ultra clean, though it costs much more.